In eukaryotic cells triacylglycerols are quantitatively the most important storage form of energy. The main pathway for synthesis of triacylglycerol is believed to involve three sequential acyl-transfers from acyl-CoA to the glycerol backbone. Acyl-CoA:diacylglycerol acyltransferase (DAGAT, EC 2.3.1.20) uses fatty acyl-CoA (acyl donor) and 1,2-diacylglycerol (acyl acceptor) as substrates to catalyze the third and only committed step in triacylglycerol synthesis. DAGAT plays a fundamental role in the metabolism of cellular glycerolipids.
Until recently, it was believed that only DAGAT could carry out the final step in triacylglycerol biosynthesis. However, it has now been demonstrated that microsomal preparation of developing seeds from several plants (sunflower, Helianthus annus; castor bean, Ricinus communis; hawk's beard, Crepis palaestina) as well as microsomal preparations from yeast (Saccharomyces cerevisiae) catalyze triacylglycerol formation via the enzyme phospholipid:diacylglycerol acyltransferase (PDAT, EC 2.3.1.158, Registry Number 288587-47-3) (WO 2000060095; Dahlqvist et al., Proc. Natl. Acad. Sci. USA 97(12):6487-6492 (2000)). This enzyme differs from DAGAT by synthesising triacylglycerol using an acyl-CoA-independent mechanism. The specificity of the enzyme for the acyl group in the phospholipid varies with species, e.g., the enzyme from castor bean preferentially incorporates vernoloyl (12,13-epoxyoctadec-9-enoyl) groups into triacylglycerol, whereas that from the hawk's beard incorporates both ricinoleoyl (12-hydroxyoctadec-9-enoyl) and vernoloyl groups. The enzyme from the yeast Saccharomyces cerevisiae specifically transfers acyl groups from the sn-2 position of the phospholipid to diacylglycerol, thus forming an sn-1-lysophospholipid. It has also been shown that PDAT activity is present in vegetative tissues of Arabidopsis thaliana (Banas et al., Biochem. Soc. Trans. 28:703-703 (2000)). Furthermore, the substrate specificity of PDAT varies between species and depends on the head group of the acyl donor, the acyl group transferred and the acyl chains of the acyl acceptor (1,2-diacylglycerol).